Hydrostatically driven vehicles are known. A typical vehicle includes an engine having an output shaft connected to one or more fluid pumps. The typical fluid pump is connected to the engine through a transmission and includes the capability of varying its displacement to accommodate power demands of the vehicle. The fluid pump or pumps associated with the vehicle is connected via fluid pressure lines to various actuators and hydraulic motors around the vehicle. For example, a vehicle may have one or more hydraulic propel motors that rotate the drive wheels or rollers and move the vehicle over the terrain. These motors are powered by a pressurized flow of fluid from the pump. The speed and torque output of these motors is proportional to the flow rate of the hydraulic fluid passing through them.
An engine on a hydrostatically driven vehicle often is configured to operate in a continuous fashion at a constant speed and torque output. For this reason, the modulation of the flow rate of hydraulic fluid supplied to the various systems of the vehicle is accomplished through control of the displacement of the pump. The vehicle operator moves a control lever to effect adjustment of the pump displacement. The control lever is connected via a cable connection to a mechanism used to change and adjust the displacement of the pump. For example, a known variable displacement pump having reciprocating pistons whose stroke can be controlled by the angle of a rotating swash-plate may have the cable operatively arranged to change the angle of the swash-plate in response to a change in position of the control lever by the operator.
While this mode of control for hydrostatic vehicles is effective, operation of the engine at this constant condition does not provide fuel economy, reduced noise, or longevity of service for the vehicle. That is, operation of the vehicle's engine at a constant engine speed and torque capability becomes wasteful when the vehicle is not operating at full load capacity.